Biaxially oriented, laminated polyester film

ABSTRACT

Disclosed is a biaxially oriented, laminated polyester film having a lamination structure with at least two layers. At least one of the outermost layers of the film is a thin laminated layer A having a thickness of not more than 3 μm. In the layer A, internally formed particles and specified agglomerated particles or mono-dispersed particles are present, or the mono-dispersed particles are present at a very small content. When the polyester film is used as a base film for magnetic recording media, the abrasion resistance of the film surface at a high speed can be greatly increased.

This application is a divisional of Application Ser. No. 08/297,611,filed Aug. 29, 1994, now U.S. Pat. No. 5,532,047.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a biaxially oriented, laminatedpolyester film particularly suitable as a base film for video tapes.

2. Description of the Related Art

As a biaxially oriented polyester film used as a base film for magneticrecording media, a film containing spherical silica particlesoriginating from colloidal silica is known and described in, forexample, JP-A-SHO 59-171623. Further, as a biaxially oriented, laminatedpolyester film, a polyester film, wherein a thin layer containingparticles to form surface protrusions is laminated on a base layer, isalso known and described in, for example, JP-A-HEI 2-77431.

In such conventional polyester films, however, there is a problem that afilm surface, particularly, protrusions formed on the film surface, islikely to be abraded by a contacting roll or guide or an edge of anapplicator, particularly accompanying with recent increase of a tape orfilm running speed in FF and REW modes of a video tape recorder, acalendering or applying process for making magnetic recording media, adubbing process, or a process for incorporating a tape into a cassette.Further, there is a problem that the signal/noise ratio (S/N) when thefilm is processed into a video tape is not so good and it is difficultto obtain a sufficiently high-quality image when the original video tapeformed from the film is dubbed particularly at a high dubbing speedwhich has been realized by a high-speed magnetic field transferringtechnology.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a biaxially oriented,laminated polyester film whose surface is hardly abraded even in ahigh-speed running in a video tape recorder or in a high-speed processfor processing the film as described above, and to provide a biaxiallyoriented, laminated polyester film which is suitable as a base film forvideo tapes wherein reduction of an image quality at the time of dubbingcan be suppressed when the film is formed as a video tape, that is, agood electromagnetic conversion property can be obtained.

To accomplish this object and other objects, a biaxially oriented,laminated polyester film according to the present invention has alamination structure with at least two layers, and is characterized inthat at least one of outermost layers of the film is a thin laminatedlayer A having a lamination thickness of not less than 0.01 μm and notmore than 3 μm. At least the thin laminated layer A contains internallyformed particles (that is, non-incorporated particles). The thinlaminated layer A may contain agglomerated particles having a meandegree of agglomeration of 5 to 100 at a content of 0.01-2% by weight,each of the agglomerated particles being formed from particles having amean primary particle diameter of 5 to 200 nm.

Another biaxially oriented, laminated polyester film according to thepresent invention has a lamination structure with at least two layers,and is characterized in that at least one of outermost layers of thefilm is a thin laminated layer A having a lamination thickness of notless than 0.01 μm and not more than 3 μm, and the thin laminated layer Acontains mono-dispersion particles having a mean degree of agglomerationof less than 5 and a mean primary particle diameter of not less than0.05 μm and not more than 3 μm at a content of not less than 0.05% byweight and less than 0.3% by weight.

When such a biaxially oriented, laminated polyester film is used as abase film for magnetic recording media, the abrasion resistance of thefilm surface at a high speed can be greatly increased.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be explained in more detail by embodimentsand examples. However, the present invention is not restricted by theembodiments and examples.

We have found that an abrasion test determined using a razor at a highfilm-running speed is suitable as an index for determining whether afilm surface is easily abraded or hardly abraded in a high-speedprocess. Further, we have also found that a low-speed razor abrasiontest wherein a film is run on a razor at a low speed can become an indexfor determining the abrasion resistance of the film surface although itis slightly poor in accuracy as compared with the high-speed razorabrasion test. The low-speed razor abrasion test can be performed moresimply and safely. The methods for performing these tests are describedlater in more detail.

As the result of investigation of the abrasion resistance of variousfilms by the above-described two tests, it has been found that filmshaving the following characteristics can achieve the objects of thepresent invention. Namely, in order to achieve a level at which theabrasion resistance in a high-speed process is no problem, the leveldetermined by the above-described high-speed razor abrasion test must benot more than 180 μm. Further, in the low-speed razor abrasion test, thelevel must be not more than 40 μm. The methods for achieving theselevels of not more than 180 μm in the high-speed razor abrasion test andnot more than 40 μm in the low-speed razor abrasion test are classifiedschematically into the following two methods.

A first method is a method for forming a lamination portion of abiaxially oriented laminated film as a thin layer and generatinginternally formed particles in the thin laminated layer. A second methodis a method for adding external particles at a very small content intothe above thin laminated layer with the internally formed particlestherein to form surface protrusions and thereby enlarging the intervalsof the protrusions. In any case, the polyester film according to thepresent invention must have a lamination structure with at least twolayers. If the film is a single-layer film, it is impossible ordifficult to control both surfaces in optimum conditions different fromeach other. In the present invention, by forming as a laminationstructure with two or more layers, for example, the surface of the layerA can be controlled to a good slipping surface and the surface ofanother outermost layer can be controlled to a flat surface. In a caseof a lamination structure with three or more layers, an intermediatelayer can be composed of a recycled polymer to contribute to save orrecycle natural resources.

Further, in the present invention, the thickness of at least one of theoutermost layers of the film (layer A) must be not less than 0.01 μm andnot more than 3 μm. By forming such a thin laminated layer A, particlespresent in the layer do not sink deeply into the interior of the filmand protrusions having a uniform height can be formed on the filmsurface.

The above-described first method will be explained in detail. Abiaxially oriented polyester film used as a base film of a video tape isgenerally added with particles to form protrusions on the surfacethereof in order to reduce the friction of the film surface. Among theprotrusions thus formed, excessively large protrusions are likely to bebroken in a high-speed process. The fragments generated by the break ofthe protrusions cause dropout at the time of recording or regenerating avideo tape. As materials forming such excessively large protrusions,particularly large mono-dispersed particles among added particles,agglomerated particles formed by agglomeration of added particles andcontaminated materials in a polymer preparing process can be raised.Further, when external particles such as spherical silica particles areused as the particles to be added to a film, because they are poor inaffinity with the matrix polymer forming the film, voids are liable tobe generated around the particles. Therefore, when an impact is appliedto the protrusions formed, the particles present beneath the protrusionsare likely to fall off.

Accordingly, in order to prevent such defects, protrusions are formed byinternally formed particles. The internally formed particles are verygood in affinity with the base material, and can form surfaceprotrusions while achieving a condition of strong bonding with the basematerial. By containing such internally formed particles in the thinlaminated layer A, broad protrusions having a uniform height can beformed on the film surface. In this case, a small amount of externalparticles may be used together with the internally formed particles, asneeded. The protrusions formed by the external particles, which arepresent between the protrusions formed by the internally formedparticles, are protected in a high-speed process by the broadprotrusions formed by the internally formed particles, and as a result,the abrasion of the protrusions formed by the external particles and thefalling of the external particles can also be prevented. Thus, externalparticles other than internally formed particles may be contained in thethin laminated layer A. The abrasion resistance at high speed and theelectromagnetic conversion property of the film can further improved byusing other appropriate particles together.

As the external particles added to the layer A, agglomerated particles(A) having a mean degree of agglomeration of 5 to 100 each of which areformed from particles having a mean primary particle diameter of 5 to200 nm, preferably 10 to 100 nm, are preferred. As the content thereof,not more than 2% by weight, preferably 0.05 to 2% by weight, morepreferably 0.1 to 1% by weight is preferred. If the content is more than2% by weight, there is a possibility that excessively large particleshaving a too large degree of agglomeration are generated. As the kindsof the particles, for example, alpha-type, gamma-type, delta-type ortheta-type alumina particles, zirconia particles, silica particles andtitanium particles can be used. These particles substantially do notform protrusions on the film surface, and are added mainly in order toreinforce the skin of the surface.

As the particles added for the purpose of reducing the friction of thefilm surface by forming protrusions among external particles added tothe layer A, mono-dispersed particles (B) having a mean degree ofagglomeration of less than 5, which have a mean primary particlediameter of 0.05 to 3.0 μm, preferably 0.1 to 2 μm, more preferably 0.2to 1 μm, can be used. As the kinds of such particles (B), calciumcarbonate particles, mono-dispersed silica particles, divinylbenzeneparticles, silicone particles, titanium oxide particles, silicon carbideparticles, silicon nitride particles, titanium nitride particles and tinoxide particles can be raised. The content thereof is preferably notmore than 3% by weight, more preferably in the range of 0.01 to 3% byweight, still more preferably in the range of 0.05 to 3% by weight, andfurther more preferably in the range of 0.05 to 2% by weight. If thecontent is less than 0.01% by weight, the friction is likely toincrease. If the content is more than 3% by weight, non-dispersed andagglomerated particles are likely to be present, and they are likely tocause an abrasion in a high speed process.

In these mono-dispersed particles (B), the mean primary particlediameter "d" (μm) of the mono-dispersed particles and the thickness "t"(μm) of the thin laminated layer A preferably satisfies the equation 0.2d≦t≦10 d, and more preferably satisfies the equation 0.3 d≦t≦5 d.

Next, the above-described second method will be explained in detail. Inthis method, the internally formed particles are not used, and only theexternal particles are used. Although the internally formed particleshave various advantages as aforementioned, there is a case where theelectromagnetic conversion property slightly deteriorates because theyform broad protrusions. In a case where a particularly high-levelelectromagnetic conversion property is required, only the particles forforming protrusions which belong to the above-described particles (B)may be added to the thin laminated layer A without using the internallyformed particles. In this case, the content of such particles relativeto the thin laminated layer A is preferably not less than 0.05% byweight and less than 0.3% by weight, more preferably not less than 0.1%by weight and less than 0.3% by weight. Although the aforementionedparticles (A) may be used to reinforce the skin of the film surface alsoin this case as needed, it is desired that the content thereof iscontrolled preferably not more than 1% by weight, more preferably notmore than 0.7% by weight, further more preferably not more than 0.5% byweight.

In the film according to the present invention, mainly with respect tothe formation of the protrusions on the layer A, the parameter ofsurface roughness (P10) is preferably not more than 350 nm, morepreferably not more than 300 nm. If the parameter is more than thisvalue, there is a fear that the abrasion resistance at high speed andthe electromagnetic conversion property decrease.

Further, with the height of the surface protrusions of the layer A, therelative standard deviation of height distribution thereof is preferablynot more than 1.2, more preferably not more than 1.0. If the relativestandard deviation is more than this value, there is a fear thatundesired excessively large protrusions are generated, such protrusionportions are likely to be abraded and thereby reducing the abrasionresistance at high speed, as well as the electromagnetic conversionproperty decreases.

Furthermore, in the film according to the present invention, the othersurface layer B is not particularly restricted, and the constitution ofthe layer B can be decided depending upon the requirements for thesurface of the layer B. For example, in a case where the surface of thelayer B is required to be formed as flat as possible, the content ofparticles (external particles and/or internally formed particles) inthis layer may be suppresses small. In such a manner, the surface of thelayer A can be formed to have an excellent abrasion resistance at highspeed, to form surface protrusions having uniform and desired height andto have an excellent electromagnetic conversion property, and at thesame time, the surface of the layer B can be formed as a required flatsurface.

In the polyester film according to the present invention, at least oneof the layers constituting the film must be biaxially oriented. It isparticularly preferred that all layers of the lamination structure arebiaxially oriented. If all layers are non-oriented or uniaxiallyoriented, the characteristics aimed by the present invention cannot besatisfied.

Although the polyester constituting the film according to the presentinvention is not particularly restricted, a polyester containing atleast one structural unit of ethylene terephthalate, ethyleneα,β-bis(2-chlorophenoxy)ethane -4,4'-dicarboxylate and ethylene2,6-naphthalate as its main constituent is particularly preferred. Amongthese, a polyester containing ethylene terephthalate as its mainconstituent is particularly preferred. Two or more kinds of polyestersmay be blended and a copolymerized polymer may be used, unless theadvantages according to the present invention are damaged.

In the film according to the present invention, other polymers may beblended as long as the objects of the present invention are not injured.Further, a generally used organic additive such as oxidation inhibitor,thermal stabilizer, lubricant and ultraviolet absorbent may be added bythe volume usually employed.

The intrinsic viscosity (IV) of the polymer of the layer A of thelaminated polyester film according to the present invention ispreferably in the range of 0.4 to 0.9 to obtain a further excellentabrasion resistance at high speed. Further, the difference between thepolymer IV of the layer A and the polymer IV of the layer B isparticularly preferably not more than 0.1 to obtain a further excellentabrasion resistance at high speed.

Next, a process for producing the film according to the presentinvention will be explained. However, the process is not particularlyrestricted by the following one.

Firstly, as a process for preparing internally formed particles, thefollowing process is effective. Namely, the internally formed particlesare prepared by adding at least one of a calcium compound, a magnesiumcompound, a manganese compound and a lithium compound soluble to glycol,and preferably, a phosphate and/or an ester compound in an appropriatemanner (a) in a process of condensation polymerization after a directesterification of predetermined dicarboxylic acid and ethylene glycol or(b) in a process of condensation polymerization after atransesterification of predetermined dimethylester of dicarboxylic acidand ethylene glycol. In the addition of the compound for forming theinternally formed particles, it is preferred that at least one of acalcium compound and a lithium compound is added to the reaction systemas a glycol solution at an appropriate timing from the time when theesterification or the transesterification has substantially finisheduntil the time when the condensation polymerization has not yetproceeded so much.

Where, as the calcium, magnesium, manganese and lithium compound to beused, compounds soluble to glycol, for example, an inorganic salt suchas a halide, a nitrate and a sulfate, an organic salt such as anacetate, an oxalate and a benzoate, a hydride and an oxide, arepreferably employed, and two or more compounds may be used together. Asthe phosphoric compounds, at least one of phosphoric acid, phosphorousacid and esters or partial esters thereof can be employed.

As the method for controlling the content of particles, a method forpreparing a particle master having a high concentration by the abovemethod and diluting the particle master with a polyester whichsubstantially does not contain particles when the film is produced iseffective. In the present invention, the concentration of the particlemaster is preferably in the range of 0.5 to 5% by weight.

Next, a polyester film having a two or more layer lamination structureis formed using the pellets of polyester thus prepared. The pellets ofpolyester prepared in the above manner are blended at a predeterminedratio, and after drying, they are supplied to known extruders formelt-extruding and lamination, molten polymers are delivered out from adie in a form of a sheet and the sheet is cooled and solidified on acasting roll to form a non-stretched film. Namely, the lamination iscarried out by using two or more extruders and a die having a manifoldor a feed block for two or more layers (for example, a feed block havinga rectangular lamination portion), a molten sheet having two layers isdelivered out from a die, the delivered sheet is cooled on a castingroll to form a non-stretched film. In such a manner, it is effective toinstall a static mixer and a gear pump in the polymer path. Further, itis effective to set the temperature of the extruder for extruding thethin laminated layer A lower than the temperature of the other extruder,for example, lower by 5° to 10° C.

Next, the non-stretched film obtained is biaxially stretched andbiaxially oriented. As the stretching process, a sequentially biaxialstretching or a simultaneously biaxial stretching can be employed.Especially, a process for using a sequentially biaxial stretching,wherein firstly longitudinal stretching and secondly transversestretching are carried out, is preferred, and a process for dividing thelongitudinal stretching into three or more stages and setting the totaldraw ratio of the longitudinal stretching to 3.5 to 6.5 times isparticularly preferred. Although it is difficult to determine theoptimum temperature for the longitudinal stretching to a particulartemperature because the optimum temperature varies depending on the kindof polyester, usually setting the temperature of the first stage to 50°to 130° C. and the temperatures of other stages after the first stage toa higher temperature is effective. The stretching rate in thelongitudinal direction is preferably in the range of 5,000 to50,000%/min. The transverse stretching is usually conducted using astenter. The draw ratio of the transverse stretching is preferably inthe range of 3.0 to 6.0 times. The stretching rate in the transversedirection is preferably in the range of 1,000 to 20,000%/min, and thetemperature for the stretching is preferably in the range of 80° to 160°C. The biaxially stretched film thus formed is then heat treated. Thetemperature of the heat treatment is preferably in the range of 170° to220° C., more preferably in the range of 180° to 200° C., and the timeof the heat treatment is preferably in the range of 0.2 to 20 seconds.

Next, methods for determining the characteristics in the presentinvention and estimating the effects according to the present inventionwill be explained.

(1) Mean primary particle diameter of particles present in film

Polymer is removed from the film by plasma low temperature ashingtreatment to expose the particles. The condition of the ashing treatmentis selected such that the particles are not damaged whereas the polymeris ashed. The exposed particles are observed by a scanning electronmicroscope (SEM) and the image of the particles is processed by an imageanalyzer. The magnification of the SEM is about 2,000 to 10,000 times,and a visual field per one determination is appropriately selected fromthe range of one side length of 10 to 50 μm. Changing the portions to beobserved, the volume mean diameter "d" of the particles of not less than5,000 is determined from the diameters and volume rate thereof by thefollowing equation.

    d=Σ d.sub.1 ·Nvi

Where, d₁ is diameter of particle, and Nvi is volume rate thereof.

In a case where the particles are organic particles and the particlesare greatly damaged by the plasma low temperature ashing treatment, thefollowing method may be employed.

Using a transmission type electron microscope (TEM), the cross sectionof the film is observed at a magnification of 3,000 to 100,000 times.The thickness of a cut sample for the measurement of the TEM is set to1,000 Å, not less than 500 fields are measured changing the fields, andthe volume mean diameter "d" of the particles is determined from theabove equation.

(2) Content of particles

The film is treated with a solvent which dissolves polyester but whichdoes not dissolve the particles, and the particles are separated fromthe polyester by centrifugation. The content of the particles is definedas the ratio of the weight of the separated particles to the totalweight of the film (% by weight). Alternatively, as needed, thedetermination may be carried out by utilizing infraredspectrophotometry.

(3) Lamination thickness of film

The depth profile of the particle density is determined by X-rayphotoelectron spectrum (XPS), infrared spectroscopic analysis (IR) or aconfocal microscope while etching from the surface. In the surface ofthe surface layer of a laminated film, the particle density is lowbecause the surface is an interface, and the particle density increasesas the measuring position is apart from the surface.

In the film laminated with a layer according to the present invention,the particle density becomes the maximum at a depth I! and thereafterdecreases again. Based on this distribution curve of the particledensity, a depth II! which indicates 1/2 of the maximum of the particledensity is defined as the lamination thickness (where, II>I). Further,in a case where inorganic particles are contained, using an apparatus ofsecondary ion mass spectrum (SIMS), the ratio of the density of theelement originating from the particles present at the highest density inthe film to the density of the carbon in the polyester (M⁺ /C⁺) isdefined as the particle density ratio, and the determination isconducted from the surface of the layer A in the depth direction(thickness direction). The lamination thickness is determined in amanner similar to the above-described manner. The lamination thicknessca also be determined by observation of the cross section of the film orusing a thin film step measuring apparatus.

(4) Mean degree of agglomeration of particles

The film containing particles is cut in the direction perpendicular tothe surface plane of the film to make a very thin piece having athickness of 1,000 Å. When the thin piece is observed at a magnificationof about 100,000 times using a transmission type electron microscope(for example, JEM-1200EX: produced by Nippon Denshi Corporation, aJapanese company), a minimum particle which cannot be further divided(primary particle) can be observed. This observation is carried out on20 fields of view, the number of primary particles forming each ofagglomerated particles is counted, and the mean value of the number isdefined as the mean degree of agglomeration.

(5) Relative standard deviation of height distribution of surfaceprotrusions

The height data of the protrusions measured with a two beam and twodetector type scanning electron microscope (ESM-3200; produced byElionics Corporation) and a cross section measuring apparatus (PMS-1;produced by Elionics Corporation) by scanning them along the surface ofthe film setting the flat portion of the film surface as the base(height 0) is transferred to an image processor (IBAS-2000; produced byKarlzuis Corporation), and an image of the protrusions on the filmsurface is reconstructed on the display of the image processor. Then, acircle equivalent diameter of a protrusion is determined from the areaof each protrusion obtained by processing the protrusion portionreconstructed on the display in a binary condition, and the determinedcircle equivalent diameter is defined as a mean diameter of theprotrusion. Further, the highest value among the height data of therespective protrusion portions of a protrusion processed into a binarycondition on the display is defined as a height of the protrusion, andthis determination is repeated on the respective protrusions. Thedetermination is repeated 500 times changing measuring portions, and ina condition where the height distribution of the determined protrusionsis supposed to indicate a normal distribution whose center is on a pointof height "0", the standard deviation is determined by least squareapproximation. The relative standard deviation of the heightdistribution of the particles is determined as the ratio of the obtainedstandard deviation to the mean height of the particles. Themagnification of the scanning electron microscope is set in the range of1000 to 8000 times.

(6) Parameter of surface roughness P10

Using a photo-interference type three-dimensional surface analyser(TOPO-3D; produced by WYKO Corporation, magnification of objective lens;40-200 times, use of a high resolution camera is effective), the imageof the protrusions of the film surface is constructed on the display ofan image processor. The mean value of ten points from the highestprotrusion portions of the image constructed on the display is definedas P10. The measurement is repeated twenty times and the mean valuethereof is defined as the parameter of surface roughness P10 (unit: nm).

(7) Intrinsic viscosity of polymer (IV)

The intrinsic viscosity of polymer is determined at 25° C. usingo-chlorophenol as the solvent.

(8) Abrasion resistance at high speed (high-speed razor abrasion test)

The film is slit into a tape having a width of 1/2 inch. The tape is runby 200 m under a condition where a safety razor is pressed onto the tapeso that the knife edge is positioned lower by 0.5 mm than the level ofthe tape applied with a tension (running speed: 200 m/min, tension: 100g) in a direction perpendicular to the tape surface. The height of thepowder on the razor, which is cut off from the tape by the razor andadhered to the razor, is determined by a microscope, and the height isdefined as the abraded amount (μm). If the abraded amount is not morethan 180 μm, the abrasion resistance is determined to be "good", and ifthe abraded amount is more than 180 μm, the abrasion resistance isdetermined to be "not good".

(9) Abrasion resistance at low speed (low-speed razor abrasion test)

The film is slit into a tape having a width of 1/2 inch. The tape is runby 10 cm under a condition where a safety razor is pressed onto the tapeso that the knife edge is positioned lower by 1.0 mm than the level ofthe tape applied with a tension (running speed: 3.3 cm/min, tension: 100g) in a direction perpendicular to the tape surface. The height of thepowder on the razor, which is cut off from the tape by the razor andadhered to the razor, is determined by a microscope, and the height isdefined as the abraded amount (μm). If the abraded amount is not morethan 40 μm, the abrasion resistance is determined to be "good", and ifthe abraded amount is more than 40 μm, the abrasion resistance isdetermined to be "not good".

(10) Electromagnetic conversion property

A magnetic coating solution having the following composition is coatedwith a gravure roll on the surface of the film so that the coatedmagnetic layer is magnetically oriented, and then it is dried. Aftercalendering the film by a small test calendering machine (steelroll/nylon roll; 5 stages) at a temperature of 70° C. and a linearpressure of 200 kg/cm, the film is cured at 70° C. for 48 hours. Thefilm obtained is slit to make a pancake of a tape having a width of 1/2inch. The tape of 250 m from the pancake is incorporated into a VTRcassette to make a VTR cassette tape.

    ______________________________________                                        (Composition of magnetic coating solution)                                    (All parts are by weight.)                                                    ______________________________________                                        Co-containing iron oxide :100 parts                                           Vinyl chloride/vinyl acetate copolymer                                                                 :10 parts                                            Polyurethane elastomer   :10 parts                                            Polyisocyanate           :5 parts                                             Lecitin                  :1 part                                              Methylethylketone        :75 parts                                            Methylisobutylketone     :75 parts                                            Toluene                  :75 parts                                            Carbon black             :2 parts                                             Lauric acid              :1.5 parts                                           ______________________________________                                    

100% chromatic signal generated by a television testing wave generatoris recorded in the tape using a domestic VTR, and chromatic S/N isdetermined from the regenerated signal using a color video noisemeasuring apparatus (unit: dB).

EXAMPLES AND COMPARATIVE EXAMPLES

Preferred examples will be hereinafter explained together withcomparative examples. The results of these examples and comparativeexamples are shown in Tables 1 to 4.

Example 1 (Table 1)

Polyethylene terephthalate pellets containing internally formedparticles of 50 parts by weight and polyethylene terephthalate pelletscontaining no particles of 50 parts by weight were blended, and theblended pellets were supplied to a vent-type twin screw extruder No. 1to melt the pellets at a temperature of 280° C. (polymer I).Polyethylene terephthalate pellets containing calcium carbonateparticles having a mean particle diameter of 0.8 μm at a content of 0.2%by weight were dried at a temperature of 180° C. for 3 hours in apressure-reduced condition (3 Torr), and thereafter the pellets weresupplied to another extruder No. 2 to melt the pellets at a temperatureof 290° C. (polymer II). After these two kinds of polymers were filteredat a high accuracy, the polymers were laminated in a feed block having arectangular lamination portion for two-layer lamination structure suchthat the polymer II is placed at a position of layer B and the polymer Iis placed at a position of layer A, and the laminated polymers weredelivered out from a fish-tail type die in a form of a sheet. Thedelivered sheet was cast on a casting drum of which surface temperaturewas controlled at 30° C. and cooled and solidified on the drum using anelectrostatic charging method to form a non-stretched film having athickness of about 160 μm. The draft ratio between the die and thecasting drum was 6.5.

The non-stretched film was stretched in the longitudinal direction atthree stretching stages wherein the first stage was at 123° C. and 1.2times, the second stage was at 126° C. and 1.45 times and the thirdstage was at 114° C. and 2.3 times. The uniaxially stretched filmobtained was stretched in the transverse direction using a stenter attwo stretching stages wherein the first stage was at 111° C. and 3.7times and the second stage was at 113° C. and 1.2 times, and then thefilm was heat treated under a condition of a constant length at 200° C.for five seconds to make a film having a thickness of 14 μm. The ratioof lamination thicknesses of the obtained biaxially oriented film was2/12 (μm) in the ratio of Layer A/layer B. The parameter of surfaceroughness (P10) thereof was 150, and the relative standard deviation ofheight distribution of the protrusions formed on the film surface was0.8.

As the result of the estimation of the properties of the film and thevideo tape made therefrom, as shown in Table 1, the electromagneticconversion property was 3.0 dB, the abrasion resistance at high speed ofthe surface of the layer A was 110 μm, and the abrasion resistance atlow speed thereof in the low-speed razor abrasion test was 15 μm. Thus,when the the kind of the particles present in the layer A and thelaminated structure satisfy the conditions specified by the presentinvention, excellent electromagnetic conversion property and abrasionresistance at high speed can be achieved.

EXAMPLES 2-5, Comparative Example 1 (Table 1)

Biaxially oriented polyester films were prepared in a manner similar tothat of Example 1. In Comparative Example 1, however, the film was asingle layer film. In Examples 2-5, external particles belonging to theparticles (A) according to the present invention were added to thelaminated layer A in addition to the internally formed particles. In anyof Examples 2-5 which satisfy the conditions specified by the presentinvention, excellent electromagnetic conversion property and abrasionresistance at high speed could be obtained.

EXAMPLES 6-12 (Table 2)

Biaxially oriented, laminated polyester films were prepared in a mannersimilar to that of Example 1. In these Examples, external particlesbelonging to the particles (B) according to the present invention wereadded to the laminated layer A in addition to the internally formedparticles. In any of Examples 6-12 which satisfy the conditionsspecified by the present invention, excellent electromagnetic conversionproperty and abrasion resistance at high speed could be obtained.

EXAMPLES 13-16, Comparative Examples 2-4 (Table 3)

Biaxially oriented, laminated polyester films were prepared in a mannersimilar to that of Example 1. In these Examples and ComparativeExamples, both of external particles belonging to the particles (A) andbelonging to the particles (B) according to the present invention wereadded to the laminated layer A in addition to the internally formedparticles. In any of Examples 2-5 which satisfy the conditions specifiedby the present invention, excellent electromagnetic conversion propertyand abrasion resistance at high speed could be obtained. However, inComparative Examples 2-4 which do not satisfy any of the conditionsspecified by the present invention, any of or all of the electromagneticconversion property and abrasion resistance obtained were poor.

EXAMPLES 17-23, Comparative Example 5 (Table 4)

Biaxially oriented, laminated polyester films were prepared in a mannersimilar to that of Example 1. In these Examples and Comparative Example,internally formed particles were not present in the layer A, butexternal particles belonging to the particles (B) according to thepresent invention were added to the laminated layer A at a smallcontent. In any of Examples 17-23 which satisfy the conditions specifiedby the present invention, excellent electromagnetic conversion propertyand abrasion resistance at high speed could be obtained. However, inComparative Example 5 which does not satisfy the condition of contentspecified by the present invention, the electromagnetic conversionproperty and abrasion resistance obtained were both poor.

                                      TABLE 1                                     __________________________________________________________________________                                              Relative                            Laminated layer A                         standard   Abrasion                                                                           Abrasion            Intern-  Particle A                                                                            Mean                                                                              Particle B                                                                         Mean                                                                              Lam-   Parameter                                                                          deviation                                                                           Electro-                                                                           resistance                                                                         resistance          ally     diamerter                                                                             degree                                                                            diamerter                                                                          degree                                                                            ination                                                                              of   of height                                                                           magnetic                                                                           at                                                                                 at low              formed   (μm) of  (μm)                                                                            of  structure                                                                            surface                                                                            distribution                                                                        conversion                                                                         speed                                                                              speed of            part-    content agglom-                                                                           content                                                                            agglom-                                                                           Thickness                                                                            roughness                                                                          of    property                                                                           layer                                                                              layer A             icles    (wt %)  eration                                                                           (wt %)                                                                             eration                                                                           (μm/μm)                                                                      t/d                                                                             P10  protrusions                                                                         (dB) (μm)                                                                            (μm)             __________________________________________________________________________    Example 1                                                                          present                                                                           --      --  --   --  A/B  --                                                                              150  0.8   3    110  15                                                2/12                                            Example 2                                                                          present                                                                           zirconia                                                                              20  --   --  A/B  --                                                                              190  0.9   2.1  130  18                           (primary) 0.04       2.5/12                                                   (secondary) 0.2                                                               0.3                                                                  Example 3                                                                          present                                                                           δ-alumina                                                                       18  --   --  A/B  --                                                                              130  0.7   3.2  120  12                           (primary) 0.02       1.5/12.5                                                 (secondary) 0.1                                                               0.3                                                                  Example 4                                                                          present                                                                           δ-alumina                                                                       18  --   --  A/B  --                                                                              180  1.1   1.6  150  21                           (primry) 0.02        1.5/12.5                                                 (secondary) 0.1                                                               1.5                                                                  Example 5                                                                          present                                                                           δ-alumina                                                                       85  --   --  A/B  --                                                                              160  1.1   1.5  160  23                           (primary) 0.02       1.8/12.5                                                 (secondary) 0.16                                                              0.3                                                                  Compar-                                                                            present                                                                           --      --  --   --  Single                                                                             --                                                                              360  1.5   0    200  41                  ative                         layer                                           Example 1                                                                     __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                                              Relative                            Laminated layer A                         standard   Abrasion                                                                           Abrasion            Intern- Particle A                                                                         Mean                                                                              Particle B                                                                           Mean         Parameter                                                                          deviation                                                                           Electro-                                                                           resistance                                                                         resistance          ally    diamerter                                                                          degree                                                                            diamerter                                                                            degree                                                                            Lamination                                                                             of   of height                                                                           magnetic                                                                           at                                                                                 at low              formed  (μm)                                                                            of  (μm)                                                                              of  structure                                                                              surface                                                                            distribution                                                                        conversion                                                                         speed                                                                              speed of            part-   content                                                                            agglom-                                                                           content                                                                              agglom-                                                                           Thickness                                                                              roughness                                                                          of    property                                                                           layer                                                                              layer A             icles   (wt %)                                                                             eration                                                                           (wt %) eration                                                                           (μm/μm)                                                                       t/d                                                                              P10  protrusions                                                                         (dB) (μm)                                                                            (μm)             __________________________________________________________________________    Exam-                                                                             present                                                                           --   --  calcium                                                                              1.01                                                                              A/B   0.72                                                                             370  1.4   1    165  37                  ple 6            carbonate  1.8/12.5                                                           2.5                                                                           0.03                                                         Exam-                                                                             present                                                                           --   --  calcium                                                                              1.03                                                                              A/B   1.36                                                                             220  1.1   1.6  140  28                  ple 7            carbonate  1.5/12.5                                                           1.1                                                                           0.05                                                         Exam-                                                                             present                                                                           --   --  calcium                                                                              1.03                                                                              A/B/A 1.25                                                                             160  0.9   2.5  130  26                  ple 8            carbonate  1.0/12/1.0                                                         0.8                                                                           0.5                                                          Exam-                                                                             present                                                                           --   --  monodispersed                                                                        1.02                                                                              A/B   1.33                                                                             120  0.8   3.2  80   18                  ple 9            spherical  0.4/10                                                             silica                                                                        0.3                                                                           2.5                                                          Exam-                                                                             present                                                                           --   --  divinyl-                                                                             1.1 A/B   1.33                                                                             110  0.6   3.4  80   16                  ple 10           benzene                                                                       0.3                                                                           1.5                                                          Exam-                                                                             present                                                                           --   --  calcium                                                                              1.02                                                                              A/B/A 2.27                                                                             370  1.3   1.1  150  31                  ple 11           carbonate  2.5/9/2.5                                                          1.1                                                                           0.3                                                          Exam-                                                                             present                                                                           --   --  calcium                                                                              1.02                                                                              A/B/A 1.82                                                                             320  1.3   2    160  18                  ple 12           carbonate  2.0/9/2.0                                                          1.1                                                                           0.3                                                          __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                                              Relative                            Laminated layer A                         standard   Abrasion                                                                           Abrasion            Intern-  Particle A                                                                           Mean                                                                              Particle B                                                                         Mean                                                                              Lam-    Parameter                                                                          deviation                                                                           Electro-                                                                           resistance                                                                         resistance          ally     diamerter                                                                            degree                                                                            diamerter                                                                          degree                                                                            ination of   of height                                                                           magnetic                                                                           at                                                                                 at low              formed   (μm)                                                                              of  (μm)                                                                            of  structure                                                                             surface                                                                            distribution                                                                        conversion                                                                         speed                                                                              speed of            parti-   content                                                                              agglom-                                                                           content                                                                            agglom-                                                                           Thickness                                                                             roughness                                                                          of    property                                                                           layer                                                                              layer A             cles     (wt %) eration                                                                           (wt %)                                                                             eration                                                                           (μm/μm)                                                                      t/d                                                                              P10  protrusions                                                                         (dB) (μm)                                                                            (μm)             __________________________________________________________________________    Exam-                                                                              present                                                                           δ-alumina                                                                      --  divinyl-                                                                           1.1 A/B/A                                                                              2.22                                                                             150  0.8   3.1  130  21                  ple 13   (primary) 0.02                                                                           benzene  1.0/12/1.0                                                (secondary) 0.1                                                                          0.45                                                               0.3        0.3                                                       Exam-                                                                              present                                                                           zirconia                                                                             20  divinyl-                                                                           1.1 A/B  3.33                                                                             120  0.6   3.5  90   21                  ple 14   (primary) 0.04                                                                           benzene  1.5/12.5                                                  (secondary) 0.2                                                                          0.45                                                               0.3        0.3                                                       Exam-                                                                              present                                                                           δ-alumina                                                                      18  calcium                                                                            1.2 A/B  1.25                                                                             140  0.6   2.8  120  30                  ple 15   (primary) 0.02                                                                           carbonate                                                                              1.0/6.0                                                   (secondary) 0.2                                                                          0.8                                                                0.3        0.5                                                       Exam-                                                                              present                                                                           δ-alumina                                                                      25  mono-                                                                              1.03                                                                              A/B/A                                                                              5.00                                                                             130  0.6   1.5  130  13                  ple 16   (primary) 0.02                                                                           dispersed                                                                              1.0/8/1.0                                                 (secondary) 0.2                                                                          spherical                                                          0.3        silica 0.2                                                                    0.5                                                       Compar-                                                                            present                                                                           θ-alumina                                                                      25  calcium                                                                            1.02                                                                              A/B  0.44                                                                             390  1.6   0.1  250  60                  ative    (primary) 0.06                                                                           carbonate                                                                              2.5/10                                           Exam-    (secondary) 0.5                                                                          1.1                                                       ple 2    2.5        3.5                                                       Compar-                                                                            present                                                                           δ-alumina                                                                      25  calcium                                                                            1.03                                                                              A/B/A                                                                              0.81                                                                             420  1.5   -0.8 250  56                  ative    (primary) 0.02                                                                           carbonate                                                                              2.5/8/2.5                                        Exam-    (secondary) 0.2                                                                          3.1                                                       ple 3    0.3        0.5                                                       Compar-                                                                            present                                                                           θ-alumina                                                                      25  calcuim                                                                            1.02                                                                              A/B  4.55                                                                             355  1.6   -2   220  48                  ative    (primary) 0.06                                                                           carbonate                                                                              5.0/8                                            Exam-    (secondary) 0.5                                                                          1.1                                                       ple 4    0.3        3.5                                                       __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                                              Relative                            Laminated layer A                         standard   Abrasion                                                                           Abrasion            Intern-  Particle A                                                                           Mean                                                                              Particle B                                                                         Mean                                                                              Lam-    Parameter                                                                          deviation                                                                           Electro-                                                                           resistance                                                                         resistance          ally     diamerter                                                                            degree                                                                            diamerter                                                                          degree                                                                            ination of   of height                                                                           magnetic                                                                           at                                                                                 at low              formed   (μm)                                                                              of  (μm)                                                                            of  structure                                                                             surface                                                                            distribution                                                                        conversion                                                                         speed                                                                              speed of            parti-   content                                                                              agglom-                                                                           content                                                                            agglom-                                                                           Thickness                                                                             roughness                                                                          of    property                                                                           layer                                                                              layer A             cles     (wt %) eration                                                                           (wt %)                                                                             eration                                                                           (μm/μm)                                                                      t/d                                                                              P10  protrusions                                                                         (dB) (μm)                                                                            (μm)             __________________________________________________________________________    Exam-                                                                              none                                                                              --     --  calcium                                                                            1.03                                                                              A/B/A                                                                              1.25                                                                             160  0.8   2.1  150  22                  ple 17              carbonate                                                                              1.0/8/1.0                                                            0.8                                                                           0.25                                                      Exam-                                                                              none                                                                              --     --  divinyl-                                                                           1.08                                                                              A/B/A                                                                              2.22                                                                             140  0.7   2.5  140  25                  ple 18              benzene  1.0/12/1.0                                                           0.45                                                                          0.25                                                      Exam-                                                                              none                                                                              --     --  divinyl-                                                                           1.08                                                                              A/B/A                                                                              6.67                                                                             240  0.9   2.6  160  25                  ple 19              benzene  2.0/10/2.0                                                           0.3                                                                           0.25                                                      Exam-                                                                              none                                                                              --     --  calcium                                                                            1.12                                                                              A/B/A                                                                              0.18                                                                             220  1.1   1.8  130  28                  ple 20              carbonate                                                                              0.2/10/0.2                                                           1.1                                                                           0.02                                                      Exam-                                                                              none                                                                              --     --  calcium                                                                            1.06                                                                              A/B/A                                                                              0.27                                                                             230  1.1   2    120  31                  ple 21              carbonate                                                                              0.3/10/0.3                                                           1.1                                                                           0.01                                                      Exam-                                                                              none                                                                              --     --  calcium                                                                            1.02                                                                              A/B  0.72                                                                             340  0.8   0.8  170  36                  ple 22              carbonate                                                                              1.8/12.5                                                             2.5                                                                           0.01                                                      Exam-                                                                              none                                                                              --     --  mino-                                                                              1.03                                                                              A/B/A                                                                              0.89                                                                             130  0.7   3.3  120  12                  ple 23              dispersed                                                                              0.4/12/0.4                                                           spherical                                                                     silica 0.45                                                                   0.25                                                      Compar-                                                                            none                                                                              --     --  calcium                                                                            1.2 A/B  0.91                                                                             380  1.6   -0.3 300  53                  ative               carbonate                                                                              1.0/6.0                                          Exam-               1.1                                                       ple 5               0.6                                                       __________________________________________________________________________

Although preferred embodiments and examples of the present inventionhave been described herein in detail, it will be appreciated by thoseskilled in the art that various modifications and alterations can bemade to these embodiments and examples without materially departing fromthe novel teachings and advantages of this invention. Accordingly, it isto be understood that all such modifications and alterations areincluded within the scope of the invention as defined by the followingclaims.

What is claimed is:
 1. A biaxially oriented, laminated polyester filmhaving a lamination structure comprising two outermost surfaces and atleast two layers, wherein at least one of said surfaces comprises a thinlaminated layer A having a lamination thickness of not less than 0.01 μmand not more than 3 μm, said thin laminated layer A containingmono-dispersed particles having a mean degree of agglomeration of lessthan 5 and a mean primary particle diameter of not less than 0.05 μm andnot more than 3 μm at a content of not less than 0.05% by total weightof said film and less than 0.3% by total weight of said film.
 2. Thebiaxially oriented, laminated polyester film according to claim 1,wherein the mean primary particle diameter of said mono-dispersedparticles is in the range of 0.1 to 2 μm.
 3. The biaxially oriented,laminated polyester film according to claim 2, wherein the mean primaryparticle diameter of said mono-dispersed particles is in the range of0.2 to 1 μm.
 4. The biaxially oriented, laminated polyester filmaccording to claim 1, wherein the content of said mono-dispersedparticles in said thin laminated layer A is not less than 0.1% by weightand less than 0.3% by weight.
 5. The biaxially oriented, laminatedpolyester film according to claim 1, wherein the mean primary particlediameter "d" (μm) of said mono-dispersed particles and the thickness "t"(μm) of said thin laminated layer A satisfies the equation 0.2 d≦t ≦10d.
 6. The biaxially oriented, laminated polyester film according toclaim 5, wherein the mean primary particle diameter "d" (μm) of saidmono-dispersed particles and the thickness "t" (μm) of said thinlaminated layer A satisfies the equation 0.3 d≦t≦5 d.
 7. The biaxiallyoriented, laminated polyester film according to claim 1, wherein aparameter of surface roughness (P10) of said thin laminated layer A isnot more than 350 nm.
 8. The biaxially oriented, laminated polyesterfilm according to claim 7, wherein the parameter of surface roughness(P10) of said thin laminated layer A is not more than 300 nm.
 9. Thebiaxially oriented, laminated polyester film according to claim 1,wherein a relative standard deviation of height distribution ofprotrusions formed on the surface of said thin laminated layer A is notmore than 1.2.
 10. The biaxially oriented, laminated polyester filmaccording to claim 9, wherein the relative standard deviation of heightdistribution of protrusions is not more than 1.0.
 11. The biaxiallyoriented, laminated polyester film according to claim 1, wherein anabrasion of the surface of said thin laminated layer A determined usinga razor at a low film-running speed is not more than 40 μm.